1. Field of the Invention
The present invention is related to a photothermographic material which is used advantageously in the fields of films for medical diagnosis and films for photoengraving.
2. Description of the Related Art
Reduction of waste solutions to be treated has been strongly desired in recent years in the medical field from the viewpoints of environmental protection and space saving. Under such circumstances, technologies on photothermographic image-recording materials as films for medical diagnosis and photoengraving which can be exposed to light efficiently with a laser image setter or a laser imager, and can form a clear black image having high resolution and sharpness have been demanded. With these photosensitive photothermographic photographic materials, it is possible to supply to customers a heat development treatment system which has eliminated the necessity of using solvent system processing chemicals, and is simpler and does not impair the environment.
The similar requirements also exist in the field of general image forming materials. However, the image for medical use is required to have a high image quality excellent in sharpness and graininess, because fine details of the image are required. In addition, the medical image is characterized by preferably exhibiting a blue black image tone from the viewpoint of ease of medical diagnosis. Currently, various hard copy systems utilizing pigments or dyes such as inkjet printers and apparatuses for electrophotography are prevailing as general image forming systems. However, there is no system which is satisfactory as a medical image-output system.
On the other hand, thermal image forming systems utilizing organic silver salts are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, as well as in “Thermally Processed Silver systems” (Imaging Processes and Materials), Neblette, 8th edition, written by D. Klosterboer, edited by J. Sturge, V. Warlworth, and A. Shepp, Chapter 9, page 279 in 1989. Particularly, the photothermographic material generally comprises a photosensitive layer in which a catalytically active amount of photocatalyst (for example, a silver halide), a reducing agent, a silver salt capable of being reduced (for example, an organic silver salt) and, optionally, a toner for controlling the tone of developed silver image dispersed in a matrix of a binder. The photothermographic material, when heated to high temperature (for example, 80° C. or higher) after imagewise exposure, forms black-toned silver images by oxidation/reduction reaction between a silver salt capable of being reduced (functioning as an oxidizer) and a reducing agent. The oxidation/reduction reaction is promoted by a catalytic activity of latent images of silver halide formed by exposure. Accordingly, black-toned silver images are formed in an exposed region.
Such photothermographic materials have been already known. However, in many recording materials, the image-forming layers are formed using an organic solvent such as toluene, methyl ethyl ketone, or methanol as a solvent. It is not advantageous to use an organic solvent as a solvent since the organic solvent may cause harmful effects on human during production process of the recording materials, and since it is costly to collect the solvent and to conduct other related processes.
In order to solve such problems, a method has been proposed in which water-based coating liquid is used for forming an image-forming layer (hereinafter sometimes referred to as “water-based photosensitive layer.” For example, techniques of using gelatin as a binder are disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 49-52626 and 53-116144, the disclosures of which are incorporated herein by reference. Further, a technique of using polyvinyl alcohol as a binder is disclosed in JP-A No. 50-151138, the disclosure of which is incorporated herein by reference.
However, these techniques are not practically satisfactory since the fogging is significant and the tone of the formed image is not good. On the other hand, techniques of using polymer latex binder and water-based medium for forming an image-forming layer are disclosed in JP-A Nos. 10-10669 and 10-62899, the disclosures of which are incorporated herein by reference.
It has been shown, for example in JP-A No. 2002-303953 (the disclosure of which is incorporated herein by reference), that processing fragility and image stability in storage in the dark (fogging at storage) can be improved by using a polymer latex with a specific physical properties as a binder. Further, JP-A No. 11-84573 (the disclosure of which is incorporated herein by reference) discloses that a low Dmin and a high Dmax are realized by using a specific polymer latex as the binder for the image-forming layer and the protective layer.
However, the performance of the photothermographic material is still unsatisfactory even when such polymer latexes are used. In particular, the image storage stability is a problem unique to photothermographic materials, and improvement thereof has been requested.